In many cases it is difficult to accurately measure the VSWR of an antenna
system that is already installed on a tower without physically climbing the tower and
sweeping the antenna on its own. This is due to the "masking effect" that
the long length of transmission line creates between the antenna at the top and the
transmitter below.VSWR of the antenna system
measured directly at the point of input to the antenna system will always be higher than
when measured with the transmission line and antenna connected in series, due to the loss
inherent in all transmission line. It is this loss that "masks" the true
VSWR of an antenna system once installed, as the reflected signal in the transmission line
is subject to the same loss characteristics on frequency in the return path coming back
down the tower as the main carrier in the forward path going up. Thus, the
forward-to-reflected ratio is always lower (worse VSWR) at the antenna because the
reflected signal has not yet been attenuated through the coaxial transmission line, as is
the case when metered at the input of the line and antenna connected in series. The
higher the transmission line loss, the higher the masking effect the line produces.
Use the following to
accurately find out the efficiency of an antenna system already installed by removing the
"masking effect" of the transmission line:
